The present invention relates generally to a method and apparatus for monitoring the electrical characteristics of a resonant cavity and, more particularly, to a method and apparatus for automatically tracking and monitoring the resonant frequency and absorbtion level of a high frequency resonant cavity that is perturbed by a material introduced into the resonant cavity.
It is well known that when a beam of high frequency energy is passed through a dielectric material, a portion of the energy will be absorbed, causing a reduction in level of the energy emerging from the material. The amount of attenuation is dependent, at least in part, on the real and imaginary parts of the dielectric constant of the dielectric material. Any changes in the dielectric constant will produce changes in the attenuation. It is also known that when a beam of high frequency energy is directed against a magnetic material, a portion of the energy will be absorbed causing a reduction in the energy emerging from the material, the reduction in energy depending, at least in part, on the real and imaginary parts of the magnetic permeativity of the material. Any changes in the magnetic permeativity of the material will produce changes in amount of energy absorbed. In addition, it is known that when a beam of high frequency energy is directed into a high frequency resonant cavity that is perturbed by either a dielectric material or a magnetic material, the material will produce a change in the resonant frequency of the resonant cavity and a reduction in the level of the outgoing beam, both of these parameters being dependent on the electrical characteristics of the perturbing material.
In recent years, a number of different systems making use of these principles have either been proposed or actually put into use for the purpose of monitoring the characteristics or changes in the characteristics of a sample material. One particular application for these systems have been in measuring the moisture content of tobacco or other water containing materials such as fuel oil.
Although the prior art systems have proven to be more or less adequate for the particular purposes for which they have been designed, these systems have not proven to be entirely satisfactory for all applications. One of the fundamental limitations of some of the prior art systems is that the measurements are made in a relatively slow manner. Another fundamental shortcoming of some of the prior art systems has been that they have generally involved some form of manual adjustment, either in connection with maintaining the resonant cavity at resonance or in adjusting some component that is used to measure the attenuation losses produced by the perturbing material. Another limitation of some prior art systems has been that they are not applicable for use with continuously moving test pieces. Other prior art systems monitor either the resonant frequency or the null depth at resonance but not both properties.
In many industrial process control systems it would be very helpful if one could accurately, quickly and automatically monitor changes in the electrical characteristics of a material. For example in the production of certain semiconductor materials it would be desirable to be able to quickly and automatically penetrate electrical signals corresponding to changes in the dielectric constant of a constituent present in a material as it is moving from one station to another. Such signals could then be used in either a feed forward or a feedback arrangement to make appropriate changes in the amount of the constituent that is present in the material.
It is the general purpose of this invention to provide a system which overcomes the aforementioned limitations by accurately, quickly and automatically generating signals representative of changes in the resonant frequency and absorbtion at resonance of a resonant cavity as it is perturbed by a material.
U.S. Pat. No. 2,798,197 discloses a microwave bridge circuit in which a sample is situated in one bridge arm and the other bridge arm is provided with a calibrated attenuator and phase shifter which are varied to obtain a balanced condition. The attenuator and phase shifter values are then used to determine the dielectric constant of the sample.
In U.S. Pat. No. 3,360,721 there is disclosed a microwave measurement system designed to detect the moisture content of tobacco. Frequency modulated microwave energy from an oscillator at a level set by a manually adjustable variable attenuator is coupled to a microwave transmitter or horn. The transmitted energy passes through a tobacco sample holder to a microwave receiver also in the form of a horn. A manually adjustable precision variable attenuator follows the receiver and couples the received energy to a detector. The precision variable attenuator is set to provide a maximum detected signal in the absence of a tobacco sample in the holder. Subsequent introduction of a sample causes a reduction in detected signal which is then compensated for by a change in the setting of the precision variable attenuator. This change in setting equals the attenuation added to the system by absorbtion of the microwave energy by the tobacco sample and corresponds to the moisture content of the tobacco.
In U.S. Pat. No. 3,460,031 there is disclosed a split waveguide moisture probe wherein the moisture content of a paper web is determined by passing the web through the gap between the probe sections. A variable attenuator at the waveguide output is used to determine the increased waveguide attenuation resulting from the passing paper web. This increased attenuation provides a measure of the web moisture content. Also disclosed in an alternate system for detecting web moisture using a split waveguide resonator probe, wherein, insertion of the web into the resonator gap changes the resonant frequency and Q of the resonator. These changes in these quantities are measured and utilized to determine web moisture content directly or as a crosscheck against an attenuation measurement.
In U.S. Pat. No. 3,498,112 there is disclosed a system for determining the moisture content of fuel oil in which energy from a microwave power source is split, with one portion of the energy being coupled through an attenuator circuit and the other portion through a sample holder containing no sample. The attenuator circuit is set so that its output when compared in a ratio meter with the output energy from the empty sample holder gives a value of 1. Subsequently placement of an oil sample in the holder, provides a ratio reduction which can then be related to the oil moisture content.
In U.S. Pat. No. 3,766,471 there is disclosed a method and apparatus for determining the moisture content of freshly prized remoistened tobacco compressed into hogs-heads. An alternating voltage is applied to a pair of electrodes connected in a circuit and thrust into the hogs-head. A phase sensitive voltmeter connected to the electrode circuit measures the component of the current in that circuit which is in phase with the alternating voltage, and hence the resistive component of the impedance between the electrodes. A logarithmic converter transforms this resistive component of the impedance between the electrodes into a signal varying linearly with moisture content.
In U.S. Pat. No. 3,77,258 there is disclosed a moisture measuring system wherein tobacco moving along a conveyor belt is compressed with a force several times the force of the dead weight of the tobacco. The compressed tobacco has an electric field applied through a first and second electrode positioned on the same side thereof. The electrodes constitute a capacitor which is part of a tuned circuit and under resonance conditions the signal derived from the tuned circuit has a magnitude corresponding to the percentage of moisture in the tobacco.
In U.S. Pat. No. 3,795,984 there is disclosed a system wherein measuring capacitor has a capacitance which varies with moisture content of the material. The capacitor is part of a tuned circuit which is kept in resonance by changing the frequency applied thereto. A voltage having an amplitude corresponding to the change in frequency constitutes a measuring signal indicative of moisture content in a high moisture content in a high moisture content range. A rectified signal derived from the voltage or current in the tuned circuit constitutes a measuring signal indicative of moisture content in the low moisture content range.
Other known patents relating to moisture measurement using high frequency energy are British Pat. No. 1,124,461, British Pat. No. 1,376,747 and U.S. Pat. No. 3,913,012.
Other known patents involving the use of high frequency energy for measuring thickness of materials are U.S. Pat. No. 3,522,527, U.S. Pat. No. 4,075,555, and British Pat. No. 993,050.